The present invention relates to a method of manufacturing a heat transfer device such as an automobile radiator from a flat copper alloy tube made of folded strip. The tube is sealed to prevent leakage during the brazing operation with a brazing paste or brazing foil inserted between the joining faces of the tube.
Many methods for manufacturing heat exchange tubes exist in the art. Most of the methods involve folding a sheet of metal to form channels, applying a flux material to the folded metal, and then heating the folded metal and flux material while applying a brazing material. During the heating process, the flux material cleans the surfaces of the metal so that the brazing material can readily flow into any gaps between the folds to seal the gap and form joints. Generally, the entire sheet of metal is coated with the flux materials and the brazing material is applied later, or the entire sheet is coated with both the brazing and flux materials. Some metals or metal alloys that are difficult to braze, such as aluminum, must be cladded with a more readily brazable metal or alloy prior to use to facilitate this type of manufacturing process.
Some examples of this technology can be found in the patent literature. European Patent Application No. 0 302 232 discloses a heat exchange tube wherein the terminal edges of the sheet material are rolled towards the center of the material past vertical so that the edges are parallel with the sheet material when they are brazed thereto. U.S. Pat. No. 4,633,056 discloses a method for manufacturing a heat exchange tube having an oval cross-section and a cross web for such tubes. The tube is joined using electron beam welding. U.S. Pat. No. 5,186,251 discloses a heat exchange tube with double row flow passages. U.S. Pat. No. 5,441,106 discloses a heat exchange tube that includes a plurality of internal fins that extend along the length of the tube. The tube is formed of cladded aluminum billet and brazed together. U.S. Pat. No. 5,579,837 discloses a heat exchange tube having a partition formed by two legs having an angle of about 7xc2x0 to 15xc2x0 between them. The entire tube is coated with brazing flux prior to brazing. U.S. Pat. No. 5,704,423 discloses a heat exchange tube produced by assembling a main portion and a secondary portion of two different pieces of metal, each generally aluminum or an aluminum alloy. U.S. Pat. No. 5,765,634 discloses a heat exchange tube divided in two by a reinforcing partition. The partition consists of a pleat extending into the interior of the tube and formed in the sheet metal strip from which the tube is fabricated. Prior to joining the ends of the metal strip, one face of the strip is coated with braze metal. As can be seen from the foregoing, no one shape is universally accepted for radiator tube manufacture. These references would generally use a flux or paste containing flux to braze together the metal, especially when using aluminum.
The flux material is generally very corrosive to the tube material and must be removed after the brazing process is complete. This adds the extra step and expense to the process to ensure the flux is removed after brazing. It is often desirable to manufacture a heat exchange tube without the need for cladding or the application of a flux. One such fluxless copper alloy used as a brazing filler material is described in U.S. Pat. No. 5,378,294. It may be useful to use such an alloy as a brazing filler material in the manufacture of heat exchanger tubing.
Despite these techniques, there still is a need for improved heat exchanger tubing formation processes, and the present invention discloses a preferred process, which avoids the disadvantages of the known techniques.
The invention relates to a method of forming a tube for a heat exchanger which comprises providing a sheet of a metal or metal alloy that has a base and two ends; folding the ends of the sheet to form legs having sides that oppose one another and sides. that oppose the base of the sheet; further folding the ends of the sheet toward one another to form a pair of fluid passageways; applying a brazing material that can adhere to the sheet material without flux between the opposing sides of the legs and between the base and the sides of the legs that oppose the base of the sheet; and applying heat to the sheet and brazing material sufficient to melt the brazing material and have it adhere to the legs and the base to join the legs to one another and to the base of the sheet to form the tube.
In this method, the sheet is advantageously made of copper or a copper alloy and the brazing material comprises a copper alloy that is formulated to have a lower melting temperature than that of the sheet. The sheet is preferably heated to no more than about 20% above the melting temperature of the brazing material, which is generally in the form of a paste or foil. When a foil is used, the brazing material has a thickness of between about 0.01 to 0.05 mm and is applied by placement between the opposing sides of the legs and between the base and the sides of the legs that oppose the base of the sheet.
When a paste is utilized, it includes a metal or alloy powder filler, a binder, and a carrier. The powder filler preferably has a particle size of between 15 and 30 microns, and is preferably an alloy of copper, nickel, tin, and phosphorus. The most preferred alloy is one having about 1% to 5% nickel, about 15% to 20% tin, about 4% to 7% phosphorus, and copper. Of course, this material can also be used for the composition of the foil, since this material can bond to copper alloys without the need for flux.
When the carrier comprises a liquid, such as water or an alcohol, the paste brazing material can be applied by brushing, spraying, or dispensing. Advantageously, the tube is dried after the spraying or brushing of the paste brazing material onto the sheet to volatilize the carrier. Alternatively, the carrier can be a wax and the binder can be a thermoplastic material with the paste brazing material being heated during application to the sheet. For this embodiment, the method further comprises melting the wax carrier and thermoplastic material binder prior to applying the brazing material onto the legs and base of the sheet, followed by cooling of the sheet to fix the brazing material in position.
A preferred application of the method is for manufacturing an automobile radiator by preparing a plurality of heat exchanger tubes according to the invention disclosed herein and then and forming the tubes into an automobile radiator. The radiator that is made by this method is yet another embodiment of the invention.